Both types of tire valve comprise a central valve core, which itself comprises a valve body incorporating a seat and a spring-loaded valve element co-acting with that seat, screwed as a plug into a tubular core holder. The valve core is a replaceable item and it and the core holder have become standardized throughout the world insofar as their cross-sectional dimensions are concerned.
In the case of snap-in valves the core holder is bonded to an elastomeric outer base so shaped that, on insertion into the valve aperture in the rim of a conventional vehicle wheel, its retention in, and airtight seal with the rim, is effected. In the case of the clamp-in type, the core holder is flanged at its lower end, threaded externally and by means of a rubber grommet, rubber and metal washers and clamping nut, may be secured air-tightly in the rim's valve aperture. In each case the core holder and associated components referred to, except for the core, are known as a valve stem.
It is well known that tire life and vehicle safety depend largely on tires being maintained at the recommended inflation pressure, which varies with the vehicle-tire combination, tire location on the vehicle and operating conditions.
Thus gauges are available which may be applied to the open upper end of the core holder of a tire valve which, by depressing the spring-loaded valve element of the valve core, become pressurised and indicate the pressure in the tire.
It is also well known that such gauges often become inaccurate with use, that motorists frequently take a risk on their vehicle's tire pressures being correct rather than go to the trouble of finding and using a gauge, and that occasionally a sealed valve commences to leak following the disturbance caused by use of a gauge.